Are you wishing to withdraw your application or the sponsorship?? You can withdraw your undertaking as long as the immigrant visa has not been issued. Once the visa is issued you cannot change or withdraw your undertaking in any way.
As for the application, you can withdraw it up until the applicant has physically landed - I do believe. [but if the visa has been issued into their passport, I think you are responsible for them if they do enter Canada within the 3 following years....]
Here is something I found on the net, that is more descriptive:
If you change your mind about sponsoring your spouse, common-law partner, conjugal partner or dependent children, you must write a letter to CPC-M before they are issued permanent resident visas (see What Happens Next? for information on the application process).
Once permanent resident visas are issued, the promise you, and if applicable, your co-signer, made to support your family is valid for the term of your undertaking.
If you decide to withdraw your sponsorship after the CPC begins processing the application for permanent residence, you will not receive a refund. If CIC has already issued a visa, or your relative or family member has already become a permanent resident, you cannot withdraw your sponsorship; you are obliged to support your relative or family member.
